A&A 512, A30 (2010) Astronomy DOI: 10.1051/0004-6361/200912079 & c ESO 2010 Astrophysics Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms F. Dulieu1,L.Amiaud1,, E. Congiu1, J.-H. Fillion1,,E.Matar1,A.Momeni1,, V. Pirronello2, and J. L. Lemaire1 1 LERMA-LAMAp, Université de Cergy-Pontoise, Observatoire de Paris, ENS, UPMC, UMR 8112 du CNRS, 5 mail Gay Lussac, 95000 Cergy Pontoise Cedex, France e-mail: [email protected] 2 Dipartimento di Metodologie Fisiche e Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Sicily, Italy Received 16 March 2009 / Accepted 4 December 2009 ABSTRACT Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic analogue of grain surface in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe the production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains,which is believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe. Key words. astrochemistry – ISM: molecules – dust, extinction – methods: laboratory – molecular processes 1. Introduction It was suggested many years ago that interstellar dust grains act as catalysts (Oort & van de Hulst 1946; van de Hulst 1946). Water, the spring of life (Brack 2002), is the most abundant Starting from simple atoms or molecules such as H, O, C, N, molecule in biological systems, and it is almost certainly of ex- CO, grains are believed to be chemical nanofactories on which traterrestrial origin. Water has been detected in gaseous or solid more complex molecules are synthesized, leading eventually to form in numerous astrophysical environments such as planets, prebiotic species produced concurrently by surface reactions and comets, interstellar clouds and star forming regions where strong by UV photons and cosmic rays irradiation, as already shown maser emissions are observed (Ehrenfreund et al. 2003; Dartois long ago (Hagen et al. 1979; Pirronello et al. 1982). The most 2005). Amorphous water ice was directly detected in dark inter- volatile species may be released in the gas phase upon formation stellar clouds through infra-red absorption (Leger et al. 1979). (Garrod et al. 2007), while the refractory ones remain on the During the formation of stars deep inside molecular clouds, gas grain surface, building up a so called “dirty icy mantle”, and may and dust become part of the infalling material feeding the cen- at least partially be sputtered by the heavy component of cosmic tral object. Part of these gas and dust grains, covered with icy rays (Johnson et al. 1991). Such mantles, which typically have a mantles (mainly composed of water), ends up in the rotating thickness of a hundred monolayers, are mainly composed of wa- disks surrounding young stars and forms the basic material from ter, the most abundant solid phase species in the Universe. Under which icy planetesimals and later planets are formed together dark cloud conditions, the very first monolayer has to grow on with comets in the external regions (van Dishoeck 2004). While bare silicate or carbonaceous grains (Papoular 2005). For most the means of delivery of water to Earth remain a subject of of the water molecules, they should be synthesized on a surface debate (Morbidelli et al. 2000), the synthesis of water in the mainly composed of water. Studying the growth of water on wa- Universe is a fundamental link in establishing our origins. Water ter ice substrate is therefore of high relevance. molecule formation in the gas phase is not efficient enough to reproduce the observed abundances in dark clouds, especially Chemical models that include water formation on grain sur- in its solid form (Parise et al. 2005; Ceccarelli et al. 2007). faces were proposed years ago by Tielens & Hagen (1982). They Therefore water ice must form directly on the cold interstellar suggested that H2O formation would be initiated by H-atoms grains and not as a condensate after being formed in the gas reacting with O, O2 and O3, although the O3+H pathway was phase. A complete review of the processes involved both in the considered the most effective and O2 wouldplayamerecat- gas and solid phase has been recently published (Tielens 2005). alytic role. Recent Monte Carlo simulations (Cuppen & Herbst 2007) show that while the main route to water formation on Present address: LCAM, Orsay, France cosmic dust grains in diffuse and translucent clouds is the reac- Present address: Université Pierre et Marie Curie - Paris 6, tion H + OH, in dense clouds the principal source of H2Oisthe LPMAA, UMR 7092 du CNRS, France reaction between H2 and OH. This study also emphasizes the Article published by EDP Sciences Page 1 of 5 A&A 512, A30 (2010) non-negligible contribution from the H + H2O2 reaction (H2O2 it an amorphous solid water ice substrate, on which wa- being a product of the H + O2 pathway) and the unusual high ter formation is studied, is prepared in two steps. First, a abundance of reactants such as OH and O3. Interestingly, an- 100-layer film of non-porous ASW ice is deposited at 120 K other code by Parise (2004) proposed a water formation scheme (Kimmel et al. 2001), then an overlayer of 10 mono-layers 15 2 where O3 molecules react with H- or D-atoms to form OH (1 ML = 10 molecules/cm ) of porous ASW ice is grown at or OD, and subsequently the reaction H2 + OH/OD leads to 10 K. The underlying non-porous water film isolates the ice layer H2O/HDO. It should be noted that this scheme was in part also from the copper substrate (Engquist et al. 1995). This double constrained by the observed abundances of deuterated species. ASW ice film is annealed to 90 K prior to each experimental run In dense clouds, observations show that hydrogen in the to avoid any further collapse of the pores between 10 and 80 K in −3 gas phase is mainly in molecular form (H/H2 ∼ 10 , Li the subsequent temperature-programmed desorption (TPD) ex- & Goldsmith 2003) and astrochemical models assume that periments (see below). In fact, the species D2,OandO2 used H-atoms are present on the surface of interstellar dust grains as in this experiment will be thoroughly evaporated by 80 K and the dominant reactive partner. A great many theoretical papers that is before any rearrangement in the porous structure of the and subsequently several experiments have accordingly treated ice template takes place. the H+H reaction and other hydrogenation reactions on the sur- The annealed water ice substrate is still porous (Kimmel face of ice mantles in molecular clouds (see, e.g., Watanabe & et al. 2001) and mimics an amorphous ice processed by UV Kouchi 2008, for an exhaustive review). The number density of and cosmic rays (Palumbo 2006) that is thought to constitute H-atoms is mostly ruled by the destruction of H2 due to cosmic the icy mantle of interstellar grains. By using an architecture rays and, whatever the density of the cloud, is about 1 cm−3.On with two separate channels (Pirronello et al. 1997), where each the other hand, the O/H2 ratio remains approximately constant consists of a triply differentially pumped beam line, two atomic (10−4), thus the number of atomic O, unlike H, is proportional to beams of O and D are aimed at theASWicesubstrateheldat the density of the cloud. Therefore, in a cloud with a density of 10 K. Atoms are produced by dissociation of O2 and D2 in mi- 103 cm−3,H/O is 10, but in a cloud with a density of 105 cm−3 the crowave discharges. The dissociation efficiency of the oxygen ratio H/O is around 1/10. See for example Caselli et al. (2002) beam is typically 40%, meaning that for 100 O2 molecules that 5 −3 where the H/O ratio is 1/7 for a cloud density of 10 cm .Ois initially feed the discharge, 60 O2 molecules and 80 O-atoms consequently the most abundant species in atomic form for very will finally reach the cold target. Trace quantities of residual dense clouds and it can accrete on grains and subsequently react gases (i.e., CO, N2,CO2 and H2O) were detected in the beam. with either H- or O-atoms. Great experimental attention has been paid, however, to confirm In previous laboratory works, Hiraoka et al. (1998)suc- that neither O3 nor deuterated compounds were present in the ceeded in producing water molecules from the reaction of H-and O-atom beam. The D-atom beam has a 60% dissociation effi- O-atoms initially trapped in a N2O matrix. In 2006, using the ciency, and no UV photons from the D2-discharge plasma can set-up FORMOLISM (see Sect. 2), we studied the formation reach the water ice sample. The purity of the beams was checked of water molecules by exposing a water ice substrate to D- and with a rotatable quadrupole mass spectrometer (QMS) that inter- O-atoms and O2 molecules, and simulated thus water formation cepted the incoming beam.